In the course of operation of a device, the device may emit EMI (hereinafter “electromagnetic interference”) into an electromagnetic spectrum, such as a radio frequency (hereinafter “RF”) spectrum. The RF spectrum is regulated by the FCC (Federal Communications Commission), and rules regarding the use thereof is proscribed under Part 15 of the FCC Rules (Code of Federal Regulations, Title 17, hereinafter referred to as “FCC Rules”). Under the FCC Rules, for example, a device that unintentionally or incidentally emits EMI must maintain EMI levels below a fixed, predetermined level.
In non-communications devices, the emitted EMI may nevertheless exceed the defined (by the FCC, for example) EMI levels for some frequencies. In communications devices, a conflict may exist between increasing the transmission capacity of the device on a given channel, and maintaining the EMI at acceptable levels, such as those defined by the FCC. The following Shannon equation, which gives the transmission capacity of a given channel, may illustrate a source of this conflict:
  C  =      ∫                            log          2                ⁡                  (                      1            +                                          s                ⁡                                  (                  f                  )                                                            N                ⁡                                  (                  f                  )                                                              )                    ⁢              ⅆ        f            where C is the channel capacity in bits/second, s(f) is the received signal PSD (power spectral density) in watts/Hz, and N(f) is the noise PSD in watts/Hz. The channel capacity may be improved by factors such as improved coding schemes, techniques which help to reduce the effective noise level, such as crosstalk cancellation, and increasing the received signal PSD. For a given channel with a given response, however, an increase in s(f) may result in an increase in launch power, i.e., an increase in the power of the signal injected into the channel by the device. Unfortunately, increasing the launch power may result in increasing the EMI beyond acceptable levels.